Molecular frequency standard

ABSTRACT

A molecular beam detector for a molecular beam tube frequency standard using a barium oxide molecule of the form Ba138O16 having spinless atoms and using electrostatic state selection, which detector comprises a heated elongated member stretched along the axis of the beam tube and arranged to intercept beam particles over a focal region extending along the length of the said member. Ions are formed in the vicinity of the elongated member and are collected by electrode means maintained negative with respect to said member.

nited States Patent Hellwig [15] 3,693,008 1 Sept. 19,1972

1541 MOLECULAR FREQUENCY STANDARD [72] Inventor: Helmut W. Hellwig, Oakhurst, NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Army [22] Filed: March 17, 1970 [21] Appl. No.: 24,966

Related US. Application Data [62] Division of Ser. No. 721,776, April 16, 1968,

Pat. No. 3,578,968.

52 US. Cl ..2s0/41.3, 331/94 51 Int Cl ..G0ln 27/78, H015 1/00 [58] Field of Search .;.....250/4l.3; 331/3, 94

[56] References Cited UNITED STATES PATENTS 2,808,510 10/1957 Norton ..331/94X 3,430,131 2/1969 Dryden ..250/4l.3X

BEAM

OTHER PUBLICATIONS Some New Applications and Techniques of Molecular Beams by J. G. King et a]. from Advances in Electronics and Electron Physics, vol. 8, 1956, pages 46- 48.

Primary Examiner-William F. Lindquist Attorney-Harry M. Saragovitz, Edward J. Kelly, I-lerbert Berl and Daniel Sharp 1 ABSTRACT A molecular beam detector for a molecular beam tube frequency standard using a barium oxide molecule of the form Ba"0 having spinless atoms and using electrostatic state selection, which detector comprises a heated elongated member stretched along the axis of the beam tube and arranged to intercept beam particles over a focal region extending along the length of the said member. Ions are formed in the vicinity of the elongated member and are collected by electrode means maintained negative with respect to said member.

2 Claims, 3 Drawing Figures OUTPUT PATENTEDSEPIQ I912 v 3.693.008

INPUT 3 OIO QQO IIIOOIOO O OQ T O UT34 F] IT Ill OVEN l2 7 FIRST RESONATOR SECOND HOT WIRE HEATER COIL l4 ENERGY STATE CAVITY ENERGY STATE DETECTOR AND SELECTOR SELECTOR MULTIPLIER F I G I ELECTRODES ELECTRODES ou'r u TNVEN TOR BY 4% ,A Mm

6W EM 0- KW ATTORNEYS HELMUT w. HELLWIG MOLECULAR FREQUENCY STANDARD This application is a division of my prior copending parent application, Ser. No. 721,776, filed Apr. 16, 1368, now US. Pat. No. 3,578,968, issued May 18, l 71.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION A molecular beam tube frequency standard offers advantages over atomic beam resonant devices, as described in the present application of Helmut W. Hellwig, Ser. No. 721,776, tiled Apr. 16, 1968, entitled Molecular Frequency Standard. Furthermore, the aforesaid parent application ennumerates the advantages accruing from the use of a barium oxide molecule of the form Ba O having spinless atoms.

One conventional method of detecting atomic beams is by stretching the detector or ribbon perpendicular to the direction of propagation of the beam. The multipole beam optics used in the present molecular beam tube display axial symmetry and the beam cross section is circumferential. Consequently, a transverse wire does not intercept a substantial portion of the beam and the detector is relatively insensitive. The detector of the subject invention includes a hot wire ionizer stretched along the beam axis. This ionizer may include a single wire, but preferably comprises two wires twisted together and separated at one end, thus permitting entrance of the molecular beam and facilitating mounting of the wire ionizer. The axial molecular beam eventually becomes convergent and the focal point is located on the wire. Practical beam tubes, however, are characterized by beam velocity variations which cause the various beam particles to be deflected from the axis of the tube by different amounts. Molecules of lower velocity, for instance, will have lower kinetic energy and will be deflected from the axis by a greater amount than molecules of higher velocity. The amount of deflection also depends upon the voltage of the state selector. This beam velocity variation thus gives rise to aberration or a defocusing effect and thus requires a multi-focal detector. In other words, a focal region exists along the axis of the beam the length of which is proportional to the velocity spread of the various beam particles. With the axial arranged wire ionizer of the invention, therefore, substantially all beam particles of the selected state will be focused on the ionizer, with greatly increased efficiency and sensitivity.

When a beam of barium oxide molecules is used the impingement of such molecule on the hot wire ionizer causes a reduction to barium and oxygen atoms. The oxygen acts as a catalyst to provide a reaction wherein the barium atoms are converted into barium ions and electrons. This catalysis greatly assists in ion generation and increases the detector sensitivity. The barium ions are collected by a portion of the detector structures, such as a cylinder surrounding the wire, which is negative with respect to the wire. Substantially all of the oxygen generated during impingement of the molecules on the surface of the hot wire is retained in the hot tungsten wire which oxidizes during bombardment by the barium oxide molecules. Consequently, no need exists for outgassing of the tube at the detector end.

SUMMARY OF THE INVENTION A molecular beam tube frequency standard using a barium oxide aa o molecule having spinless atoms and using electrostatic state selection. In the presence of external magnetic fields, the Zeeman effect causes symmetrical splitting of the upper energy transition level of such molecules into three sublevels, the middle one being independent of any external magnetic field and the other two being equally dependent upon the magnetic field with opposite sign. Since electrostatic (non-magnetic) state selection can be used, the two magnetic field dependent sublevels will always be equally populated; this effect, coupled with the symmetrical Zeeman splitting makes it possible to avoid frequency shift owing to external magnetic fields.

The molecular beam tube includes a beam source comprising a thin-walled refractory oven tube of iridium containing barium oxide to be evaporated. The iridium oven tube, which does not react chemically with the barium oxide, is surrounded by an electron source such as a tungsten cathode which is electrically heated. A direct current voltage is applied between the source and the oven tube such that the oven tube forms the anode; consequently, electrons from the cathode bombard the oven tube and the material in the oven tube is heated by electron bombardment, whereby a beam of molecular barium oxide is produced.

The molecular beam, after appropriate state selection, is directed through a cavity resonator which is arranged to support a TM mode. The resonator includes a cylinder with two end pieces provided with holes for passage of the beam. In order to minimize frequencyshifting distortions caused by irregularities in the cylinder-end piece junctions, each of the ends of the cylinder is provided with a sharp edge and the flat surface of each end piece is pressed against one of the sharp edges by mounting screws, assuring even contact since the sharp edge cuts slightly into the material of the respective end piece.

Detection of that portion of a molecular beam emanating from the second state selector is accomplished by a hot wire ionizer of axial symmetry. The wire ionizer is stretched along the axis of the molecular beam. The ionizer can comprise, for example, a pair of twisted wires separated at one end to permit entrance of the beam. The focal points of the converging beam can be located anywhere along the whole length of the detector wire, thus providing simultaneous detection of all velocities present in a multi-velocity beam.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing schematically a molecular beam tube according to the invention;

FIG. 2 is an exploded view showing the relationship of the principal elements of the molecular beam tube of FIG. 1;

FIG. 3 is a view of a hot wire ionizer detector according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates schematically a molecular beam tube 5 according to the invention. An exploded view of the tube, with only a portion of the envelope, is shown in FIG. 2. The molecular beam is generated by evaporation of Ba O in an oven 10 which comprises an iridium heater tube 12 containing the barium oxide in solid form. The heater tube 12 is surrounded by heater wires 14 which is negative with respect to the heater tube so that the electrons emitted from the heater wire bombard the heater tube 12 to heat the latter to the required evaporation temperature of the barium oxide. The molecular beam then enters a first state selector 15 which can be of the quadrupole type designed for optimum focusing of the molecules in the upper sublever J=l ,M= with a most .probable speed of about 4.3Xl0 centimeters per second (corresponding to an oven temperature of l,500 C.) when operated at kilovolts. As shown in FIG. 2, this quadrupole focuser or state selector comprises four equally spaced electrodes or wires 66 with alternating ones being interconnected. A high voltage is applied in push-pull to the alternate electrodes or wires which are brought out to terminals 72 angularly displaced about one of the two annular ceramic spacers 68 and 69. The molecule of the selected state are concentrated along the axis of the tube by the focusing effect produced in the state selector 15. The other states of lower energy level are deflected away from the longitudinal axis of the beam tube 5 and do not enter the cavity resonator. Input microwave energy of the proper transition frequency [8.702 GHz) is supplied to a Rabi cavity resonator by way of a waveguide 93 from any conventional microwave generator, such as a klystron, not shown. This resonator 20 is preferably designed to operate in the TM mode so as to obtain substantial interaction between the electric field within the microwave cavity resonator and the molecular particles passing through the cavity resonator. The latter is designed to minimize mechanical and electrical irregularities which tend to produce phase and Doppler shifts within the cavity resonator. The cavity resonator 20 is supported within the envelope 9 of the tube 5 by ceramic members 85 and 86. The Ba O molecules, upon leaving the cavity resonator, pass through a second electrostatic state selector or focuser (see FIGS. 1 and 2) which may be identical to the first state selector 15. The molecules of the desired J=l, M=0 state are converged by the state selector 25 onto the axis of the molecular beam tube while molecules of other energy levels are deflected away from the axis of the molecular beam tube 5. Because themolecular beam has a Maxwell velocity distribution, thesecond state selector 25 will have a focal region along the axis of the beam tube rather than a single focus. A substantial portion of the molecules of the selected state impinge upon a hot wire ionizer detector which comprises an axial wire 102 (or wires 102, 103, in the version shown in FIG. 3) heated to a relatively high temperature to increase the speed of response. A substantial portion of the barium oxide molecules impinge upon a focal region along the axially arranged wire of the detector 30 and the ions resulting from said impingement are collected for example, by cylinder 107 (see FIG. 3) to which an output lead 34 is connected. The entire detector assembly 30 includes two supporting discs 35 and 36 for mounting the detector within the envelope 9 of the beam tube 10. An output current is provided which isa minimum whenever the cavity resonator excitation coincides with the resonant transition frequency.

As previously stated, the quadrupole state selector is an efiicient means for focusing the J=l M=0 molecules of a speed, which, for the Ba O" molecule at 1,500 C., is about 4.3 centimeters per second.

The molecules of the undersired states are not focused along the longitudinal axis of the state selector but are deflected away from said axis. The amount of deflection of the undesired energy states from the beam tube axis is a function of the particular molecule, the velocity of the molecules and the length and radius of the state selector and the state selector voltage. For example, the longer the state selector, the greater the deflection; however, if the length is made too great, troubles arise owing to an increase in the number of molecular collisions.

The Ba O molecule of the proper state pass along the axis of the beam tube 5 through a narrow opening in a cavity resonator 20, which comprises essentially an elongated cylinder 82, which may be made of any hard metal having a low thermal expansion coefficient, such as invar, to minimize frequency shifts owing to thermal detuning of the resonator. The cavity resonator assembly 82 is supported within the beam tube envelope 9 by electrically insulating spacers 85 and 86 attached to the resonator subassembly. An energy coupling slot is provided in the wall of cylinder 82 and is arranged to receive a waveguide 93 extending out of the beam tube envelope 9. The portion of the waveguide external to the beam tube 5 is connected to a microwave generator such as a magnetron or klystron capable of providing reasonable amounts of energy at the transition frequency of the Ba O molecules, that is, at approximately 18.7 GHz.

After the interaction of the molecular beam of the desired energy state with the electric field in the cavity resonator, the molecular beam passes into a second electrostatic state selector or focuser 25. This state selector may be identical with the first state selector 15 already described. Again, the action of the second state selector is such as to deflect any molecules of the undesired energy level away from the longitudinal axis of the tube and causes the molecules of the desired energy to converge along said axis.

The molecules of the selected energy state now must be detected and this detection is accomplished by a detector 30 of the hot wire ionizer type. Because of the axial symmetry of all other beam tube elements, the beam focused on the detector also is of axial symmetry. The detector 30, mounted within tube 9 by end discs 35 and 36, comprises a heated tungsten wire stretched along the longitudinal axis of the beam tube. This axial mounting is facilitated if the wire actually includes at least two wires 102 and 103 twisted together. At one end the wires are spread apart so that the aperture in the end disc 35 for entrance of the molecular beam is not obstructed. The detector 30 further includes a cylindrical collector electrode 107 insulated from the end disc by discoidal glass insulating rings 108. The two separated ends of the wires 102 and 103 are mounted by means of respective mounting screws 111 and 112 to the'end disc 35. The other ends of the twisted wires 102 and 103 passthrough an opening in end disc 36 and are attached by a set screw 113 to a bushing 114 which is spring-loaded by spring 115 so as to provide the tension necessary to compensate for thermal expansiou of the heated wire and thereby maintain the twisted ionizer wires taut. 4

t The wire ionizer 102, 103 is heated to a temperature of the order of l,500 C. in order to provide a suitable detector response speed, which speed increases with wire temperature. The wire ionizer 102, 103 is connected by leads 1 16 and 117 passing through respective insulators l 18 and 119 in the wall 9 of the beam tube 5 to an external heating source 120 which can be rated for example, at about 1 volt and 5 amperes. The cylindrical ion collector electrode 107 is maintained negative with respect to the wire ionizer 102, 103 by means of an external power supply 123 of about 20 to 30 volts which is connected by leads 116 and 125 passing through bushings 118 and 127 in the wall 9 of beam tube 5 to the hot wire ionizer 102, 103 and collector electrode 107, all respectively. As previously explained, the velocity spread inherent in the molecular beam is such that the second state selector does not have a single focus but rather a focal range along the axis of the beam tube 5. By stretching the wire ionizer along the axis of the beam tube, rather than transverse to said axis, as in previous beam tubes, it is possible to achieve efficient detection along the total axial focal range of the state selector 25.

When the Ba O molecules strike the various portions of the wire ionizer 102, 103 within the focal range, they are converted into barium atoms and oxygen atoms in a first reaction and barium atoms, in turn, are converted in a second reaction into barium ions and electrons. The oxygen produced during the first reaction acts as a catalyst in increasing the amount of barium ions released from the wire 102, 103. The speed at which the first of these reactions occur increases with temperature and, although the temperature of the wire ionizer is not critical, the hotter the wire the faster the formation of barium ions. These ions are attracted to the negative collector electrode 107 and the current flows to an output terminal 34 connected to the collector 107.

Much of the oxygen produced during the first of the two reactions mentioned above will be absorbed in the oxidation of the tungsten wire and the beam tube can be a closed off system without the necessity for any oil pump at the detector end of the tube for oxygen removal.

In practice, the frequency of the microwave energy entering the cavity resonator 20 is swept at a relatively low rate and, as the frequency varies about the transition frequency of the Ba O molecule, the ion current at the collector electrode 107 of the detector will vary. The output of the detector 30 is amplified by circuitry, not shown, and is used to control a crystal oscillator, in the usual manner.

I claim:

1. The combination of a molecular beam tube and a detector for detecting a molecular beam directed along the longitudinal axis of said molecular beam tube comprising a detector assembly mounted within said molecular beam tube by a pair of spaced discs, said detector assembly including a tubular ion collector electrode fixedly mounted to said spaced discs, said detector assembly further including an elongated electrically conductive means stretched along said longitudinal axis or inter e ti earn mol cules 'r s e tiv of the ocal region 0 said beam ong said lo iigitudinal axis that is coextensive with said elongated means, said elongated means being disposed coaxially with and surrounded by said collector electrode, a bushing resiliently engaging one of said spaced discs, one end of said axial elongated means being secured to said resiliently mounted bushing to allow for changes in length of said elongated means with temperature changes, and means for heating said elongated means to a temperature sufficient to generate ions of said beam molecules upon impingement of said beam molecules upon said elongated means, said collector electrode being maintained negative with respect to said elongated means for collecting said ions formed in the vicinity of said elongated means.

2. A detector according to claim 1 wherein said elongated means comprises at least two wires spaced apart at said other end to avoid obstruction of said molecular beam. 

2. A detector according to claim 1 wherein said elongated means comprises at least two wires spaced apart at said other end to avoid obstruction of said molecular beam. 